Tech guru and YouTuber Tom Scott has posted a handy and easy to understand explanation as to why this event occurs. The quick and complicated explanation is that it is an "integer underflow caused by the Unix epoch," with Scott later on expanding this information for people like me who have no idea what 90% of those words mean.

Scott explained that setting your iPhone date to January 1, 1970 means that the value integer for this becomes 0. While this integer itself being 0 doesn't matter, when your phone tries to receive a text message and display it as being sent in the past, your phone will try and set this calendar integer into the negatives in order to explain the past-tense text message, crashing your phone.

While this explanation hasn't been proven legitimate by any official sources, it's the closest we are to understanding the issue completely.